Conventionally, the following techniques (i) and (ii) have been proposed, and have greatly advanced interdependently on at least growth in use of liquid crystal displays: (i) an integrated circuit element technique in which a single-crystal silicon substrate is processed so that approximately hundreds of millions of transistors are formed on the single-crystal silicon substrate; and (ii) a thin film transistor (TFT) technique in which a polycrystalline semiconductor film such as a silicon film is formed on a light-transmitting amorphous substrate such as a glass substrate, and is then transformed into transistors so as to be prepared as semiconductor elements, for use in a liquid crystal display device, such as picture elements, switching elements, and drivers.
Of the techniques (i) and (ii), according to the integrated circuit element technique (i), for example, a commercially available single-crystal silicon wafer having a thickness of at most 1 mm and having a diameter of approximately 200 mm is processed so that a large number of transistors are formed on the silicon wafer.
In contrast, according to the thin film transistor technique (ii), in a case where it is employed in a TFT liquid crystal display device, for example, an amorphous silicon film formed on an alkali-free glass substrate having a light-transmitting property (i.e., having an amorphous; having a high strain point) is melted and polycrystallized by heating such as the laser heating, and is then transformed into MOS transistors serving as switching elements.
Another technique has been proposed in which a silicon thin film, in particular a single-crystal silicon thin film, is formed on an insulator by a transfer method. Actually, various substrates are produced by the transfer method.
In a field of integrated circuits, a substrate is employed so as to improve a function of a semiconductor element such as a transistor.
Specifically, in a case where a transistor is prepared with the use of a substrate, the transistor (element) is completely separated from other components. This causes a reduction in constraints on the operation of the transistor. Consequently, the transistor exhibits good properties and high performance.
Note here that the substrate employed in the field of integrated circuits is simply required to be an insulator (or an insulating film). It will not matter whether or not the substrate is light transparent and it will not matter whether or not the substrate is a crystalline material.
In contrast, in a field of display devices such as a TFT liquid crystal display (LCD) device and a TFT organic electroluminescence (organic light emitting diode; OLED) display device, a substrate is required to be transparent. Hence, an amorphous substrate such as a glass plate is normally used as the substrate.
After a thin film such as an amorphous silicon film or a polysilicon film is formed on such a transparent substrate, TFTs are prepared by use of the amorphous silicon film or the polysilicon film. Such TFTs can function as switching elements for carrying out so-called active matrix driving with respect to the display device.
In order for circuits for the active matrix driving such as peripheral drivers and timing controllers are integrated on a substrate, research has been conducted on development of a higher-performance substrate on which a silicon film is formed.
Conventionally, in a case where a polysilicon film is used as a silicon film, it has been a tendency for the following problems (i) and (ii) to occur: (i) a localized level occurs in a gap due to imperfection in crystallinity; and (ii) a localized level occurs in a defective gap near a grain boundary. The occurrence of any of such localized energy levels has caused problems such as a decrease in mobility and/or an increase in subthreshold coefficient (S coefficient) in transistors. This ultimately caused a degradation in performance of the transistors.
Further, in a case where the silicon film of a polysilicon film has defective crystallinity, fixed electric charge were easily generated in an interface between the silicon film and a gate insulating film in a thin film transistor. This generation of such fixed electric charge made it difficult to control a threshold voltage of the thin film transistor, and made it difficult to achieve a desired threshold voltage.
In a case where a polysilicon film is formed by heating an amorphous silicon film by laser beam irradiation, transistors prepared from the polysilicon film greatly varied in their mobilities and their threshold voltages. This is because the size of grains in the polysilicon film that has been irradiated by laser beam is not uniform due to the fluctuation in energy level of the laser beam.
During laser heating, there occurred an instantaneous temperature rise, up to around a melting point of silicon, in a polysilicon film which was being formed by the laser heating of the amorphous silicon film. This occasionally caused an element such as an alkali metal contained in the glass substrate to be diffused in the silicon film, thereby deteriorating transistor characteristics.
In view of the circumstances, studies have been conducted on devices employing single-crystal silicon so as to address the above problems caused by employing a polysilicon film.
An example of a device employing the single-crystal silicon is disclosed in Patent Literature 1 (Japanese Patent Application Publication, Tokukai, No. 2004-134675 A; Publication Date: Apr. 30, 2004).
Specifically, Patent Literature 1 discloses a light-transmitting substrate (glass substrate) on which a coating film and a single-crystal silicon thin film are formed in this order, the single-crystal silicon thin film being divided into layers by implanting hydrogen ions into a single-crystal transferred member.
According to the conventional light-transmitting substrate, however, a problem was caused that bubbles were generated between the glass substrate and the single-crystal silicon thin film. The bubbles generated between the glass substrate and the single-crystal silicon thin film refer to minute air bubbles generated between the glass substrate and the single-crystal silicon thin film. The single-crystal silicon thin film floats above the glass substrate in a region where bubbles are generated. Thus, the single-crystal silicon thin film is not in contact with the glass substrate in such a region. This is further described below.
First, with reference to (a) through (e) of FIG. 7, the following describes a structure of a SOI substrate and a method for producing the SOI substrate as an example of the conventional substrate. (a) through (e) of FIG. 7 are cross-sectional views schematically illustrating a process of producing the SOI substrate.
As illustrated in (e) of FIG. 7, which is a cross-sectional view schematically illustrating an arrangement of the SOI substrate, a transferred substrate 170 includes a light-transmitting substrate 120 such as a glass substrate; and a single-crystal silicon thin film 150 formed on the light-transmitting substrate.
The transferred substrate 170 is generally produced as follows.
First, the following members (i) and (ii) are prepared: (i) the light-transmitting substrate 120, such as a glass plate, serving as a supporting substrate for the SOI substrate; and (ii) a single-crystal transferred member 160 (see (a) and (b) of FIG. 7).
Then, as illustrated in (c) of FIG. 7, a separatory substance is implanted into the single-crystal transferred member 160. Specifically, hydrogen ions serving as the separatory substance are implanted into the single-crystal transferred member 160 via a surface (implantation surface 162), more specifically via a substantial entirety of the implantation surface 162 (as indicated by arrows shown in (c) of FIG. 7).
The hydrogen ions, thus implanted in the single-crystal transferred member 160, reach and stay at a predetermined depth from the implantation surface 162. This allows a peeled layer 110 to be formed.
Next, as illustrated in (d) of FIG. 7, the single-crystal transferred member 160 illustrated in (c) of FIG. 7 is combined with the light-transmitting substrate 120 illustrated in (a) of FIG. 7. The single-crystal transferred member 160 is combined with the light-transmitting substrate 120 so that the implantation surface 162 is in contact with the light-transmitting substrate 120.
Then, the single-crystal transferred member 160 is divided (peeled) so that the single-crystal silicon thin film 150 is formed on the light-transmitting substrate 120.
Specifically, the light-transmitting substrate 120 with which the single-crystal transferred member 160 has been combined is heated at a high temperature such as 600° C. This heating causes a rapid cubical expansion of the hydrogen forming the peeled layer 110, and consequently generates a blister in the peeled layer 110 (more specifically, in a region of the peeled layer 110 in which region the hydrogen ions have the highest concentration). As illustrated in (e) of FIG. 7, this causes the single-crystal transferred member 160 to be separated or peeled along the peeled layer 110 serving as an interface. On this account, the transfer substrate 170 is formed which is the light-transmitting substrate 120 on which the single-crystal silicon thin film 150 is formed.
The above description deals with the case where the SOI substrate is obtained by combining the single-crystal transferred member 160 with the light-transmitting substrate 120 so that the single-crystal silicon thin film 150 is provided on the light-transmitting substrate 120. However, instead of the single-crystal transferred member 160 (semiconductor film), a semiconductor substrate (semiconductor device) on which devices and the like are formed in advance can be combined with the light-transmitting substrate 120 by use of a similar method.
The transfer substrate 170 produced by the above method, however, had at least the problem that the bubbles were generated in an interface (see IF shown in FIG. 8) between the single-crystal silicon thin film 150 and the light-transmitting substrate 120.
Specifically, bubbles 180 are generated in the interface IF between the single-crystal silicon thin film 150 and the light-transmitting substrate 120 (see FIG. 8 which is a cross-sectional view illustrating the transfer substrate 170). The single-crystal silicon thin film 150, which has been transferred onto the light-transmitting substrate 120, is floated above the light-transmitting substrate 120 in a region where the bubbles 180 are generated.
The bubbles 180 are generated due to various reasons. It is often the case that the bubbles 180 are generated during the step of heating the single-crystal transferred member 160 which has been combined with the light-transmitting substrate 120 so that the peeled layer 110 is separated from the single-crystal transferred member 160.
In other words, the bubbles 180 appear to be liable to generate during a heating step which causes the semiconductor film or the semiconductor substrate on which the device and the like is formed to be peeled along the peeled layer. The heating step is carried out after combining the semiconductor film or the semiconductor substrate on which the device and the like is formed with the light-transmitting substrate 120. It appears to be the case that the bubbles 180 are often generated due to concentration of hydrogen and/or water contained in the semiconductor film and other members occurs in a part of the transferred interface IF in which part binding energy is weak.
Any transferred substrate, in which the bubbles 180 are generated in the transfer interface IF as illustrated in FIG. 8, is incapable of being used as an intended transferred substrate. This caused a reduction in yield of producing transferred substrates.